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Centrifugal pumps come with a performance curve at the time of delivery, on which the appropriate operating range for the pump is indicated. When using this pump, the user should make adjustments to ensure it operates as much as possible within a reasonable range. There are two methods to adjust the operating conditions of a centrifugal pump: changing the performance curve of the unit and changing the performance curve of the pump. (1) Changing the performance curve of the device: The operating point of a centrifugal pump is determined by the intersection of the pump’s performance curve and the device’s characteristic curve. If one of the two curves changes, then the intersection point moves accordingly, that is, the operating condition point of the pump changes. When the piping system is set. Opening or closing the control valve on the discharge pipeline increases or decreases the resistance loss in the pipeline, and the device’s characteristic curve changes accordingly. Therefore, by adjusting the gate valve on the discharge pipeline, it is possible to easily adjust the operating conditions of the centrifugal pump. (2) Changing the pump performance curve 1) Changing the speed: The specific method is described in the section on the law of proportionality. 2) Reduce the number of impellers in the multistage pump or turn down the outer diameter of the impellers. In operation, it is common to find that the flow rate and head of some centrifugal pumps exceed the actual requirements. To ensure the efficient and economical operation of such pumps, while also maintaining a certain reserve head, efforts are made to eliminate the excess head. The excess head of a centrifugal pump cannot be simply determined by subtracting the actual required head from the rated head of that pump. It is also necessary to take into account factors such as the decline in performance due to wear of pump components, and the reduction in rotational speed caused by changes in grid frequency. The excess head can be eliminated using the following two methods: For multi-stage pumps, the impeller can be removed, and this removal should be carried out at the discharge end. Removing the impeller at the suction side can increase the resistance on that side, leading to cavitation. The segmented multi-stage pump has its intermediate section that can be removed. But the axis must be changed at this point. It is also possible to remove only the impeller of the multi-stage pump while keeping the intermediate section, thereby avoiding the need to replace the shaft; this approach only results in some increase in head loss. In multi-stage pumps and ordinary single-stage pumps where excess head is present and more than one impeller cannot be removed, the outer diameter of the impeller is often turned down to eliminate the excess head. The specific method is as follows. The relationship between the amount of cutting of the impeller and the resulting changes in its performance is as follows: (the parameters after cutting are denoted by the subscript “’”) Q’/Q = D2’/D2, that is, D2’ = D2 × (Q’/Q); H’/H = (D2’/D2)², that is, D2’ = D2 × (H’/H)¹/². P’/P = (D2’/D2)³, that is, D2’ = D2 × (P’/P)¹/³. The above formula can be used to preliminarily determine the amount of cutting required for the impeller, but the actual amount of cutting should also be determined by referring to the performance curves and the changes in performance after cutting. In general, it is necessary to carry out the cutting in several steps rather than completing it in one go, as this helps to avoid insufficient head after cutting. Example: For the EAPl00-250 model, the operating parameters are QN=220 m3/h and HN=70 m. An impeller with a diameter of 250 mm is used; according to the curve, when the flow rate is Q=220 m3/h, the head is H=73 m. At this point, the impeller needs to be cut. The diameter of the impeller after cutting can be calculated using the formula: D2’ = D2 × (H’/HN)¹/² = 250 × (70/73)¹/² = 244.8 mm. If the diameter is reduced to 244.8 mm, the performance curve will change as shown in the figure below. When the head is 70 m, the flow rate will decrease to: Q’ = QN(D2’/D2) = 220(244.8/250) = 215.4 m3/h. The performance curve after the modification will pass through the points Q and H; it can be seen from this curve that when the pump operates at its rated flow rate of Q = QN = 220 mm3/h, the head will be lower than 70 m. Therefore, the cutting amount should be reduced during cutting, and the process should be carried out in multiple steps; a cutting amount of 3 mm can be used at this stage. The flow rate and head after cutting will be: Q’ = QN(D2’/D2) = 220(247/250) = 217.4 m3/h, and H’ = HN(D2’/D2) = 73(247/250)2 = 71.26 m. According to the performance curve after cutting, when the pump operates at its rated flow rate, the head will still be higher than 70 m. After the first cut, operational data must be obtained through experiments. A second cut can be made if necessary. By cutting in this manner, excessive cutting can be avoided. 3) Grinding the back side of the impeller outlet section: By grinding the back side of the impeller outlet section, the outlet angle of the blades is increased as well as the open area between the outlets of adjacent blades. Meanwhile, the increased angle on the back side of the blade outlets helps to reduce the flow deviations and uneven velocity distributions that result from a limited number of blades. By trimming the back side of the impeller outlet, efficiency can be slightly improved; this results in an increase in pump head of about 2% to 5% at the same flow rate, and an increase in flow rate of about 5% to 10% at the same head.